The English word week continues an Old Englishwice, ultimately from a Common Germanic*wikōn-, from a root *wik- "turn, move, change". The Germanic word probably had a wider meaning prior to the adoption of the Roman calendar, perhaps "succession series", as suggested by Gothicwikō translating taxis "order" in Luke 1:8.

The term "week" is sometimes expanded to refer to other time units comprising a few days. Such "weeks" of between 4 and 10 days have been used historically in various places.[1] Intervals longer than 10 days are not usually termed "weeks" as they are closer in length to the fortnight or the month than to the seven-day week.

Seven-day week

Evidence of continuous use of a seven-day week appears with the Jews during the Babylonian Captivity of the 6th century BC.[2] Both Judaism (based on the Creation narrative in the Torah/Bible) and ancient Babylonian religions used a seven day week.[3] Other cultures adopted the seven-day week at different times. Between the 1st and 3rd centuries the Roman Empire gradually replaced the eight day Roman nundinal cycle with the seven-day week.[4] Hindus may have adopted a seven day week as early as the 1st century BC. There is evidence of some Chinese groups using a seven day week as early 4th century AD.

Systems derived from the seven-day week

Soviet Union

Between 1929 and 1931 USSR changed from the 7-day week to a 5-day week. There were 72 weeks and an additional five national holidays inserted within three of them totaling a year of 365 days.

But as January, March, May, July, August, October and December have 31 days, the week after the state rest day of the 30th was 7 days long (31st–7th). This extra day was a working day for most or extra holiday for others.

Also as February is only 28 or 29 days depending if a leap year or not, the 1st of March was also made a state rest day, although not every enterprise conformed to this.

To clarify, the week after the state rest day, 24/25 February to 1 March, was only 5 or 6 days long, depending if a leap year or not. The week after that, 2 to 6 March, was only 5 days long.

The calendar was abandoned 26 June 1940 and the 7-day week reintroduced the day after.

Decimal calendar

A 10-day week, called décade, was used in France for 9½ years from October 1793 to April 1802; furthermore, the Paris Commune adopted the Revolutionary Calendar for 18 days in 1871.

Christian "eighth day"

For early Christians, Sunday, as well as being the first day of the week, was also the spiritual eighth day, as it symbolised the new world created after Christ's resurrection. The concept of the eighth day was symbolic only and had no effect on the use of the seven-day week for calendar purposes. Justin Martyr wrote: "the first day after the Sabbath, remaining the first of all the days, is called, however, the eighth, according to the number of all the days of the cycle, and [yet] remains the first".[5] This does not set up an 8-day week, since the eighth day is also considered to be the first day of the next cycle (i.e., not the following day).